That's just cuttlebone. It mostly consists of crystalline calcium carbonate called Aragonite. It's like coral. It will not do any harm. It's just an expensive and in my eyes quite ineffective way of stabilising your pH like you do with crushed coral.
Agree
Earthworms are great for omnivorous and carnivorous species.
But if heavily fed to semi-and primarily vegetarian species (like mbuna, and Trophies,) they can be deleterious to their digestive tracts.
As far as feeding turtle bone, the same concept may apply.
Carnivorous fish, get high calcium levels when the eat other fish (bones and all).
But for the African vegetarians above, or Central American vegetarians like Cincelichthys, or South American semi vegetarians like Severum, or Uaru, that eat higher plants or scrape and eat primarily algae, they may not tolerat the elevated calcium levels if fed as a steady diet.